Hot swap decoupling for noise reduction and failure prevention

ABSTRACT

An apparatus for selectively connecting a decoupling capacitor in parallel with a load on a power bus during a hot swap power up. In an aspect, an apparatus includes a capacitive coupling connected to a hot swap control circuit and that capacitively couples first and second power conductors when the output of the hot swap control circuit indicates a monitored load has attained a voltage threshold.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of, and claims priorityto, U.S. patent application Ser. No. 13/963,624, titled “HOT SWAPDECOUPLING FOR NOISE REDUCTION AND FAILURE PREVENTION,” filed on Aug. 9,2013. The disclosure of the foregoing application is incorporated hereinby reference in its entirety for all purposes.

BACKGROUND

This specification relates to power controllers for sensing andcontrolling power during hot swaps.

A hot swap power controller facilitates the addition of electricaldevices to a system without removing power from other electrical devicesin the system. An example of the use of a hot swap power controller isin a server rack, where server devices may be added by inserting theserver devices while other server devices in the server rack and on thesame power bus remain powered. When the server device is coupled to theserver rack, and thus to the power bus, the hot swap controller monitorsthe supply voltage and other conditions, such as current, and cancontrol the power up of the server device so as to avoid transients,such as excessive inrush currents. Once the server device reaches apowered state that meets a threshold, the hot swap power controllergenerates a “power good” signal to indicate to the server processor (orother system components) that the server device is now operable.

If a hot swap controller is not used, the server device may introduce aconsiderable capacitive load to the power bus. Because the capacitiveload may be uncharged, it may demand a large inrush current to charge.The large inrush current, in turn, reduces the power bus voltage, whichmay cause brownouts among other electronic devices powered by the powerbus.

Additionally, decoupling capacitors are used to smooth the voltage on apower bus. Due to limited bandwidth, power supplies cannot respond toinstantaneous current changes that a capacitive load may introduce. Tocompensate for the changes in current, decoupling capacitors areconnected across the power bus, from the supply conductor to the returnconductor. The decoupling capacitors add a fast charge storage near theload that provides charge to the load. Accordingly, the use of thedecoupling capacitor reduces the transients in the power supply voltageresulting from changes in the load current.

SUMMARY

In general, one innovative aspect of the subject matter described inthis specification can be embodied in an apparatus that includes acapacitive load having first and second terminals, the first terminalconfigured to be in electrical connection with a first conductor of apower bus; a switching device having first and second terminals and ancontrol input, the first and second terminals being in electricalconnection when the switching device is in a closed state and being inelectrical isolation from each other when the switching device is in anopen state, wherein the first terminal of the switching device is inelectrical connection with the second terminal of the capacitive load,and the second terminal of the switching device is configured to be inelectrical connection with a second conductor of the power bus; and ahot swap control circuit configured to be in electrical connection witha load that is powered by the first and second conductors of the powerbus, and that monitors an electrical state of the load while the loadtransitions from an unpowered state to a fully powered state in responseto the load being connected to the first and second conductors of thepower bus, wherein the hot swap control circuit outputs a first signalindicative of the electrical state of the load meeting a first thresholdduring the transition from the unpowered state to the fully poweredstate, and outputs a second signal indicative of the electrical state ofthe load not meeting a first threshold during the transition from theunpowered state to the fully powered state, and the input of theswitching device receives the output of the hot swap control circuit andis in the closed state when the first signal is output by the hot swapcontrol circuit and is in the open state when the second signal isoutput by the hot swap control circuit. Other embodiments of this aspectinclude corresponding methods.

Another innovative aspect of the subject matter described in thisspecification can be embodied in an apparatus that includes a serverrack configured to receive computer servers and electrically connectingeach computer server to a power bus that includes a first conductor anda second conductor; a capacitive load having first and second terminals,the first terminal be in electrical connection with the first conductorof the power bus; a switching device having first and second terminalsand an control input, the first and second terminals being in electricalconnection when the switching device is in a closed state and being inelectrical isolation from each other when the switching device is in anopen state, wherein the first terminal of the switching device is inelectrical connection with the second terminal of the capacitive loadand the second terminal of the switching device is in electricalconnection with a second conductor of the power bus; and the controlinput is configured to be in electrical connection with an output of acontrol circuit when a computer server is received in the server rack,the control circuit in electrical connection with server and thatmonitors an electrical state of the server while the server transitionsfrom an unpowered state to a fully powered state in response to beingconnected to the first and second conductors of the power bus, whereinthe control circuit outputs a first signal indicative of the electricalstate of the server computer meeting a first threshold during thetransition from the unpowered state to the fully powered state, andoutputs a second signal indicative of the electrical state of thecomputer server not meeting a first threshold during the transition fromthe unpowered state to the fully powered state; and the switchingdevice, in response to the output of the control circuit, is in theclosed state when the first signal is output by the control circuit andis in the open state when the second signal is output by the controlcircuit. Other embodiments of this aspect include corresponding methods.

Particular embodiments of the subject matter described in thisspecification can be implemented so as to realize one or more of thefollowing advantages. The hot swap decoupling apparatus selectivelyremoves the decoupling capacitor during a hot swap event, which reducespower bus noise that would otherwise result from charging the decouplingcapacitor. However, once the hot swap controller generates the powergood signal, the decoupling capacitor is reconnected across the powerbus, which then reduces the susceptibility of the power bus to voltagefluctuations due to changes in load current resulting from neighboringhot swap events. Thus the benefit of the decoupling capacitor duringnormal load operation is realized without the drawback during the hotswap power up. This enables systems to be less dependent on individualhot swap controllers noise immunity limitations, and provides moreflexibility in power distribution design.

Some systems increase noise immunity by including an inductor (orferrite bead) in the power path. Those components are serial and add tothe constant losses during server operation. In some implementations ofthe systems described herein, no serial components are needed, so thereis no additional power loss cause during normal operation. The hot swapdecoupling apparatus can obviate the need for constant capacitors beingadded to the power distribution bus, thereby eliminating the addition ofextra load to the rectifiers. Furthermore, the effective serialresistance of the capacitor by switch can be controlled by selection ofa particular field effect transistor (FET) or selection of an explicitresistor. Such tuning allows for damping control due to the resulting RCtime constant.

The details of one or more embodiments of the subject matter describedin this specification are set forth in the accompanying drawings and thedescription below. Other features, aspects, and advantages of thesubject matter will become apparent from the description, the drawings,and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a circuit that incorporates hot swapdecoupling during a hot swap power up.

FIG. 2 is a flow diagram of an example process for hot swap decoupling.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

The subject matter of this written description relates to hot swapdecoupling and coupling of a coupling capacitor to a power bus. In someexample implementations, a decoupling capacitor can be selectivelycoupled to and decoupled from a supply and return conductors on a powerbus. Selective coupling is controlled by a power good signal from a hotswap controller that is separate from the decoupling capacitor. In theabsence of a power good signal, the decoupling capacitor is decoupledfrom the power bus. However, when the power good signal is present, thedecoupling capacitor is coupled to the power bus. In this manner, thedecoupling capacitor is not coupled to the power bus during the initialphase of a connection of an electrical device to a power bus and thepower up of the electronic device. However, once the power good signalis generated, the decoupling capacitor is coupled to the power bus, andprovides a fast voltage source to smooth voltage transients resultingfrom load current transients.

FIG. 1 is a circuit diagram of a circuit 100 that incorporates hot swapdecoupling during a hot swap power up. The circuit 100 depicts anelectrical device 120 that has been connected to a power bus thatincludes a first conductor 102 and a second conductor 104. The firstconductor provides a supply voltage, and the second conductor 104provides a return. A selective decoupling capacitor circuit 110 isconnected to the conductors 102 and 104. The selective decouplingcapacitor circuit 110 can, in some implementations, be integrated intothe electrical device 120, or, alternatively, can be integrated into arack device that receives the electrical circuit 120. The power busconductors have an inherent inductance, as represented by the inductorsin the in each conductor 102 and 104.

In some implementations, the selective decoupling capacitor circuit 110includes a capacitive load and a switching device. In someimplementations, that capacitive load is a capacitor 112, and the switchis a field effect transistor 114. The RC time constant can be selectedby including an optional resistor 113, and/or by selection of Rds(ON) ofthe field effect transistor 114. Other impedances can also be used, suchselection of additional reactive impedance by use of an inductor inserial connection with the capacitor 112.

A first terminal of the capacitor 112 is connected to the firstconductor 102 of the power bus, and a second terminal of the capacitor112 is connected to one of either the source or drain terminals of thetransistor 114. The other of the source or drain is connected to thesecond conductor 104. The transistor 114 forms a switch that, dependingon the voltage signal applied to the gate of the transistor 114,selectively couples or decouples the coupling capacitor 112 between thefirst and second conductors 102 and 104.

The electrical circuit 120 includes a hot swap control circuit 122 andother circuitry that is generally modeled as an electrical load 124. Thehot swap control circuit 122 is configured to be in electricalconnection with the electrical load 124 and monitors an electrical stateof the load 124 while the load 124 transitions from an unpowered stateto a fully powered state in response to being connected to the first andsecond conductors 102 and 104 of the power bus. The hot swap controlcircuit 122 can be any appropriate hot swap control circuit, and canmonitor the electrical state by one or more of current sense circuits,voltage sense circuits, and other sensing circuits. The hot swap controlcircuit 122 include an output 126 on which the circuit 122 outputs afirst signal indicative of the electrical state of the load meeting afirst threshold during the transition from the unpowered state to thefully powered state, and outputs a second signal indicative of theelectrical state of the load not meeting a first threshold during thetransition from the unpowered state to the fully powered state. Thefirst signal is generally referred to as the “power good” signal.

The hot swap control circuit 122 is shown in parallel with the loadcircuit 124. The topology, however, is an abstraction, and the actualconnection varies according to the particular hot swap control circuitutilized. For example, the hot swap control circuit 122 may incorporatea current limiter or other switching device in series with the load 124to control the load inrush current while the load 124 is powered up.

The gate input of the transistor 114 receives the output 126 of the hotswap control circuit 122. The transistor 114 is in the open state whenthe second signal is output by the hot swap control signal or whenfloating, and is in the closed state when the first signal, the powergood signal, is output.

Operation of the circuit 100 is described with reference to FIG. 2,which is a flow diagram of an example process 200 for hot swapdecoupling. In operation, the electrical circuit 120 is inserted intothe rack, thereby coupling the electrical circuit 120 to the power busconductors 102 and 104. The hot swap control circuit 122 monitors thepower state of the load 124 as the load 124 transitions from anunpowered state to a powered state (202). Because the load 124 isinitially unpowered, the output 126 is in the second state, indicatingthe load circuit 124 has not powered up to at least the threshold levelat which the power good signal is generated. Accordingly, the transistor114 is in an open state, and the second terminal of the capacitor 112 isnot in electrical communication with the second conductor 104 of thebus.

Eventually the load circuit 124 powers up to the first threshold level.The threshold level may be less than a fully powered state and greaterthan an unpowered state, e.g., an 80% capacitive charge of the load 124.In response to the power state of the load circuit 124 meeting thethreshold, the power good signal is generated on the output 126. Thepresence of the power good signal at the gate of the transistor 114causes the transistor 114 to act as a closed switch, which, in turn,electrically couples the second terminal of the transistor 114 to thesecond conductor 104 of the power bus. The coupling capacitor 112 isthus introduced into the circuit.

The introduction of the separate coupling capacitor 112 at this point intime will not have deleterious effects on the power supply voltagebecause, for example, the voltage has already reached a threshold level,e.g., 80% of its final value. The capacitive charge on the load 124 willassist in charging the coupling capacitor 112, which reduces the inrushcurrent required to charge the coupling capacitor. Furthermore, theabsence of the coupling capacitor 112 during power up and prior to thegeneration of the power good signal does not render the bus susceptibleto transients, as the hot swap control circuit 124 controls the power upof the load circuit 124.

The circuit 100 can be implemented in a variety of different ways. Forexample, in some implementations, a rack can be configured without anydecoupling capacitors, and the selective decoupling capacitor circuit110 and the hot swap control circuit 122 can be included in eachelectrical circuit 120. Alternatively, the selective decouplingcapacitor circuit 110 can integrated into the rack and the couplingbetween the rack and the electrical circuit 120 can be configured sothat the hot swap control circuit 122 output 126 is coupled to the gateof the transistor 114 upon insertion of the electrical circuit 120. Inyet another implementation, the selective decoupling capacitor circuit110 and the hot swap control circuit can be integrated into each rackslot that receives the load circuit 124.

Furthermore, other control circuits instead of the hot swap controlcircuit can be used to control the capacitor circuit 110. For example, ahigh impedance monitoring circuit can be connected in parallel with thehot swap control circuit and generate the output signal that controlsthe capacitor circuit 110. The output signal can be subject to athreshold voltage that is different from the threshold voltage at whichthe “power good” signal of the hot swap control circuit is generated.

While this specification contains many specific implementation details,these should not be construed as limitations on the scope of anyinventions or of what may be claimed, but rather as descriptions offeatures specific to particular embodiments of particular inventions.Certain features that are described in this specification in the contextof separate embodiments can also be implemented in combination in asingle embodiment. Conversely, various features that are described inthe context of a single embodiment can also be implemented in multipleembodiments separately or in any suitable subcombination. Moreover,although features may be described above as acting in certaincombinations and even initially claimed as such, one or more featuresfrom a claimed combination can in some cases be excised from thecombination, and the claimed combination may be directed to asubcombination or variation of a subcombination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. Moreover, the separation of various system components in theembodiments described above should not be understood as requiring suchseparation in all embodiments. Particular embodiments of the subjectmatter have been described. Other embodiments are within the scope ofthe following claims. In some cases, the actions recited in the claimscan be performed in a different order and still achieve desirableresults. In addition, the processes depicted in the accompanying figuresdo not necessarily require the particular order shown, or sequentialorder, to achieve desirable results.

What is claimed is:
 1. An apparatus, comprising: a capacitive loadhaving first and second terminals; a switching device having first andsecond terminals and an control input, the first and second terminalsbeing in electrical connection when the switching device is in a closedstate and being in electrical isolation from each other when theswitching device is in an open state, wherein the first terminal of theswitching device is in electrical connection with the second terminal ofthe capacitive load; and a hot swap control circuit that monitors anelectrical state of a load while the load transitions from an unpoweredstate to a powered state in response to the load being connected tofirst and second conductors of a power bus, wherein the load is separatefrom the capacitive load, wherein: the hot swap control circuit outputsa first signal indicative of the electrical state of the load meeting afirst threshold during the transition from the unpowered state to thepowered state, and outputs a second signal indicative of the electricalstate of the load not meeting the first threshold during the transition;and the input of the switching device receives the output of the hotswap control circuit and the switching device is in the closed statewhen the first signal is output by the hot swap control circuit toconnect the capacitive load to form a decoupling capacitor in parallelwith the load, and is in the open state when the second signal is outputby the hot swap control circuit so the capacitive load and does not forma decoupling capacitor.
 2. The apparatus of claim 1, wherein theswitching device is a field effect transistor.
 3. The apparatus of claim1, wherein the capacitive load is a capacitor.
 4. The apparatus of claim1, wherein the first threshold is a powered state that is less than afully powered state and greater than an unpowered state.
 5. Theapparatus of claim 1, further comprising a resistor in serial connectionwith the capacitor.
 6. A method, comprising: monitoring, by a hot swapcontrol circuit, an electrical state of load while the load transitionsfrom an unpowered state to a powered state in response to the load beingconnected to the first and second conductors of a power bus; in responseto the monitoring, generating, at an output conductor: a first signalindicative of the electrical state of the load meeting a first thresholdduring the transition from the unpowered state to the powered state; asecond signal indicative of the electrical state of the load not meetinga first threshold during the transition from the unpowered state to thepowered state; coupling the output conductor to a switching device thatreceives the outputs of the hot swap control circuit and is in a closedstate when the first signal is output by the hot swap control circuitand is in an open state when the second signal is output by the hot swapcontrol circuit; coupling a capacitor, by the switch, between the firstand second conductors of the power bus when the switch is in the closedstate to connect the capacitor in parallel with the load so that thecapacitor forms a decoupling capacitor in parallel with the load; anddecoupling the capacitor, by the switch, from the first and secondconductors of the power bus when the switch is in the open state so thecapacitor is not connected to the second conductor of the power buswhile the load is connected to the first and second conductors of thepower bus.
 7. An apparatus, comprising: a hot swap control circuitconfigured to be in electrical connection with a load that is powered byfirst and second conductors of a power bus, and that monitors anelectrical state of the load while the load transitions from anunpowered state to a powered state, wherein: the hot swap controlcircuit outputs a first signal indicative of the electrical state of theload meeting a first threshold during the transition from the unpoweredstate to the powered state, and outputs a second signal indicative ofthe electrical state of the load not meeting the first threshold duringthe transition from the unpowered state to the fully powered state; andmeans for connecting a capacitive coupling between the first and secondconductors of the power bus when the second signal is output to connectthe capacitive coupling parallel with the load so that the capacitivecoupling forms a decoupling capacitor in parallel with the load and fordisconnecting the capacitive coupling between the first and secondconductors of the power bus when the first signal is output so thecapacitive coupling is not connected to the second conductor of thepower bus while the load is connected to the first and second conductorsof the power bus.